Sanders usually consist of a sanding belt traveling on power driven rollers. Some belt sanders have a fence which is attached to the frame of the sander and extends up, above, and across the sanding belt to provide a stop against which the work piece (wood or other material) is placed to prevent it from being moved with the sanding belt. The operator places the work piece on the moving belt and against the fence to sand the work piece. The work piece must be sanded and then measured several times to ensure that the work piece is of the proper thickness.